To
improve the efficiency of airport security screening, the FAA deployed the
Computer Assisted Passenger Screening system (CAPS) in 1999. CAPS attempts to
identify potential terrorists through the use of profiles so that security
personnel can focus the bulk of their attention on high-risk individuals. In
this paper, we show that since CAPS uses profiles to select passengers for
increased scrutiny, it is actually less secure than systems that employ random
searches. In particular, we present an algorithm called Carnival Booth
that demonstrates how a terrorist cell can defeat the CAPS system. Using a
combination of statistical analysis and computer simulation, we evaluate the
efficacy of Carnival Booth and illustrate that CAPS is an ineffective
security measure. Based on these findings, we argue that CAPS should not be
legally permissible since it does not satisfy court-interpreted exemptions to
the Fourth Amendment. Finally, based both on our analysis of CAPS and
historical case studies, we provide policy recommendations on how to improve
air security.

On the
morning of September 11, 2001,
nineteen terrorists boarded four separate commercial airplanes across the
northeastern seaboard of the United States. Moments after takeoff, they made their moves. Using the
cardboard box cutters and razor blades they had smuggled through airport
security checkpoints, the terrorists, working in teams of four or five, started
slashing the throats of flight attendants and passengers in order to lure the
pilots out of the cockpits. Before long, the terrorists had commandeered the
controls of the aircraft, transforming their vehicles from passenger jets to
guided missiles laden with sixty thousand pounds of explosive fuel each. By
midmorning, the operation was complete. A field in rural Pennsylvania bellowed smoke. An entire facade of the Pentagon in Washington, DC lay in shambles. And the TwinTowers of the WorldTradeCenter tumbled from the Manhattan skyline. In all, thousands of people perished on that
morning. America, President Bush vowed, would never be the same.[1]

Almost
immediately, the finger pointing began between federal agencies over who was
responsible for not thwarting this attack—the most destructive ever carried out
on the United
States
mainland. Some blamed the Central Intelligence Agency (CIA), calling it the
largest intelligence failure since Pearl
Harbor. Others assailed the
Immigration and Naturalization Service (INS) for letting such dangerous
individuals enter the country in the first place. But the one agency the public
most criticized was the Federal Aviation Administration (FAA). How is it that
they let nineteen people armed with knives slip right through security
checkpoints? The grainy security camera pictures of Mohammed Atta, the
terrorist ringleader, clearing security at the airport in Providence, RI, on September 11 now haunt America’s consciousness. If only the metal detectors had been more
sensitive, if only the security personnel had been more alert, if only Atta had
aroused more suspicion, then perhaps this tragedy would never have happened.

With the
vulnerabilities of the nation’s air transportation infrastructure made
painfully clear, the FAA had a new sense of urgency in plugging the holes in a
security system that was widely recognized as being as porous as a sieve. But
the challenges in doing so were, and still are, daunting. Over 639 million
passengers pass through airports annually in the USA.[2]
Given this tremendous volume in traffic, unparalleled by any other nation,
time-consuming security screening of every passenger is unfeasible. Since the
number of security personnel available in airports is resource constrained, it
would simply take too long to search everyone as meticulously as airports are
able to in some other countries, such as Israel. While Americans love to fly, they also hate to wait.

To address
these vexing security problems, the FAA has been trying in recent years to
employ information technology to boost the overall efficiency of security
screening. The kernel idea behind their approach is to be more intelligent
about which passengers are selected for rigorous inspections. Intuitively, the
FAA argues, if you only have the ability to scrutinize a small percentage of
passengers, it seems best to spend the bulk of time carefully searching those
who are likely to be terrorists and not waste much time searching those who
have a small chance of posing harm. Why frisk Eleanor, the 80-year-old
grandmother from Texas when you can stop Omar, the 22-year-old student fresh from Libya? If you can develop a profile describing who is likely to
be a terrorist, then those are the people upon whom you should concentrate your
security efforts.

Drawing
from these intuitive underpinnings, the crown jewel of the FAA’s information
technology efforts is a system called the Computer Assisted Passenger Screening
system (CAPS). The FAA contends that since CAPS uses profiles to pinpoint
potential terrorists for closer inspection, it will not only result in the
apprehension of more criminals, but will also make security screening more
expedient for well-meaning citizens. Though in place since 1999, CAPS has
gained much more attention as a promising counter-terrorism tool in the wake of
September 11. The FAA already augmented the system in January, and plans for
further expansion are underway.[3]

In our
paper, we show that although these intuitive foundations might be compelling,
their implementation in CAPS is flawed. That is to say that any CAPS-like
airport security system that uses profiles to select passengers for increased
scrutiny is bound to be less secure than systems that randomly select
passengers for thorough inspection. Using mathematical models and computer
simulation, we show how a terrorist cell can increase their chances of mounting
a successful attack under the CAPS system as opposed to a security system that
uses only random searches. Instinct may suggest that CAPS strengthens security,
but it in fact introduces a gaping security hole easily exploitable by
terrorist cells. It should be noted that CAPS has also received immense
criticism from privacy advocates and civil libertarians[4],
but in this paper we restrict our discussion to a purely technical perspective
and the legal and policy implications of such an analysis.

In Section
2, we better define how the CAPS system operates. In Section 3, we present our
algorithm for defeating CAPS. To evaluate the algorithm’s efficacy, in Section
4 we present the results of a probabilistic analysis and computer simulation of
airport security. In Section 5, we discuss a few case studies to understand
what security techniques have been effective historically. In Section 6, we
discuss the legal implications of our finding that CAPS performs worse than
random search. And finally, we conclude in Section 7 with policy
recommendations for how to improve air security.

During the
second term of his administration, President Clinton convened a panel to
develop a set of recommendations to improve air transportation security. The
resulting White House Commission on Aviation Safety and Security (chaired by
Vice-President Gore) published its final report[5]
in February of 1997, giving the federal seal of approval to automated passenger
profiling. “Based on information that is already in computer databases,” the
Gore Commission wrote, “passengers could be separated into a very large
majority who present little or no risk, and a small minority who merit
additional attention.” This statement also serves to articulate the federally
supported two-stage architecture behind passenger profiling systems. First, the
system should develop a secret profile describing characteristics of high-risk
individuals (profile development). And then security manpower should be focused
on those individuals who, based on available data, match the profile (profile
evaluation). Such passengers are referred to as selectees.

The Gore
Commission was cognizant of the constitutional fragility of such a system, so
they invited testimony from civil liberty groups who were concerned that the
derived profile might violate Fourteenth Amendment equal protection. To pacify
these fears, the Commission dictated that “No profile should contain or be
based on material of a constitutionally suspect nature.” The Commission also
insisted that the FAA should periodically consult the Department of Justice “to
ensure that selection is not impermissibly based on national origin, racial,
ethnic, religious or gender characteristics.” Finally, to ensure that no one
group is singled out, the Commission recommended that passenger profiling
systems also choose random people who do not fit the profile as selectees.[6]

Using the
guidelines set forth by the Gore Commission, the FAA and Northwest Airlines
jointly completed development of the first version of CAPS in 1998. The federal
government closely guards the details of how the system operates, citing a
compelling national security interest— if the specifications were to be
released, it would be trivial for potential criminals to defeat the system. But
drawing from an assortment of news articles[7],
interviews with airport personnel, the Gore Commission report[8],
leaks captured on the congressional record,[9]
and prototypes written by software companies bidding to develop future versions[10],
a cohesive if not comprehensive understanding of CAPS can be painted.

CAPS
operates according to the same two-stage model described in the Gore Commission
report: profile development followed by profile evaluation. First, based on a
historical record of data pertaining to known terrorist activities, the
software attempts to detect subtle patterns in the data that correlate with
prior terrorist plots and anti-correlate with the activities of non-criminals.
For instance, the software might find that those people who bought one-way
tickets in cash and traveled abroad frequently had an elevated chance of being
terrorists. CAPS then assembles these patterns into a secret profile suitable
for inspection by the Department of Justice. Since the DOJ certification is
only held periodically, the derived master profile is presumably static for
long periods of time.

On a more
technical level, CAPS likely accomplishes pattern detection through the use of
a three-layer neural network. The first layer of the network contains hundreds
if not thousands of nodes, the third layer contains a single output node, and
the second layer contains an intermediate number of nodes. Each field of data
available for profile development is fed into a separate node of the first
layer. Using a standard training procedure, such as back propagation, the
weights of each connection are set such that when data from a terrorist is fed
into the network, the output layer returns a value close to one. But when data
from a non-criminal is fed into the network, the output layer returns a value
close to zero. If training of the network is successful, the matrix of
connection weights serves as the profile.

The CAPS
system installed in 1999 and currently in use is only capable of doing profile
development over data pertaining to the history of ticket purchases. Future
versions of CAPS, however, will be able to incorporate a richer set of data,
including driving history, credit card purchases, telephone call logs, and
criminal records, among other information. Though allowing CAPS to access some
of this data would require changes in privacy legislation, Congress, following
on the heels of the PATRIOT act, is poised to facilitate.[11]

Once CAPS
crafts a profile, it is incorporated into software that is accessible from
every airline check-in counter nationwide. When a passenger checks in, the
ticket agent enters the passenger’s name into the CAPS console. Data mining
software linked to government databases then scours for information about the
passenger, retrieving data relevant to the profile. The software compares the
similarity of the acquired data to the profile and computes a “threat index”
assessing how much potential risk that passenger may pose.[12]
In the technical scenario outlined above, the computed threat index would
simply be the product of the profile matrix with the passenger’s mined data
vector.

If the
passenger has one of the top 3-8% of threat indices relative to the other
people on his flight, then CAPS flags him for “special treatment.” To protect
the integrity of the CAPS system, the precise percentage of people flagged by
CAPS is unpublished, but it is known to be in that range.[13]
To comply with the Gore Commission guidelines, a small percentage of people on
each flight are randomly flagged as well. In all, the total number of people
flagged by CAPS is limited by the security personnel resources available at
each particular airport.

What
exactly do these “special treatment” flags entail? Before September 11, CAPS
flags were only tied to the passenger’s checked baggage, which were scanned
using costly explosives-detection equipment. This represented the conventional
wisdom of the time that a terrorist would try to smuggle explosives only in his
checked luggage[14].
But now, the FAA is tying CAPS flags to individuals. Someone flagged by CAPS
may have her carryon bags specially inspected, she may be subject to
questioning, she may be asked to stand in a separate line, she may be asked to
comply with a search of her body, or a guard may even escort her directly to
the gate.[15]

In an
article in Slate magazine, Microsoft Chief Architect Charles Simonyi
related his experience of being flagged by CAPS.[16]
During a routine business trip, security personnel insisted on completely
unpacking and repacking all of his carryon bags. This happened time after time.
”Then it hit me,” Simonyi writes. “It was not that security was especially
tight: It was only me they wanted. The label my friendly hometown airline had
affixed to my bags had unexpectedly made me a marked man, someone selected for
some unknown special treatment.” The bottom line is that if CAPS flag you,
you’ll be treated differently, and you’ll know it.

This
transparency is the Achilles’ Heel of CAPS; the fact that individuals know
their CAPS status enables the system to be reverse engineered. You, like
Simonyi, know if you’re carryons have been manually inspected. You know if
you’ve been questioned. You know if you’re asked to stand in a special line.
You know if you’ve been frisked. All of this open scrutiny makes it possible to
learn an anti-profile to defeat CAPS, even if the profile itself is always kept
secret. We call this the “Carnival Booth Effect” since, like a carnie, it
entices terrorists to “Step Right Up! See if you’re a winner!” In this case,
the terrorist can step right up and see if he’s been flagged.

We will now
present an algorithm that a terrorist cell can employ to increase their
probability of mounting a successful attack under the CAPS system as opposed to
an airport security system that employs only random searches. The key idea is
that a terrorist cell can probe the security system to ascertain which of their
members have low CAPS scores. Then they can send these members on destructive
missions. Since security manpower is disproportionately spent on people with
high CAPS scores, and the operative has a low score, he will most likely face
reduced scrutiny.

The
algorithm, which we call Carnival Booth, then is as follows: (1) Probe
the system by sending an operative on a flight. The operative has no intent of
causing harm. He has no explosives. He has no weapons. He has nothing. He
simply takes the flight and notes whether or not CAPS flags him. (2) If he is
flagged, then send another operative in the same manner. (3) Repeat this
process until a member who consistently eludes CAPS flags is found. (4) Now
send this operative on a mission with intent to harm, complete with weapons or
explosives. Since CAPS didn’t flag him last time, he likely won’t be flagged
this time, so he incurs much less risk of special scrutiny.

To better
understand how a terrorist cell using this algorithm stands a better chance of
success under CAPS than under a random system, let’s consider the numerical
example illustrated in Figure 1. Suppose an airport only has the personnel
resources to give 8% of people special scrutiny; the other 92% undergo standard
screening through a metal detector. Under a system where people are selected at
random, this airport can afford to flag 8% randomly. This means that every time
a terrorist attempts to go through security, he stands an 8% chance of
increased scrutiny. This will be true no matter what tactic or algorithm the
terrorist uses.

(Random)

(Metal Detectors)

Random System

Terrorist Activity

Increased Scrutiny

No Terrorist Activity

Standard Scrutiny

Figure 1: Regions of Terrorist Activity
under CAPS and a Random System

Now compare this to the same airport using a CAPS system,
which may for example flag the 6% of passengers with the highest threat indices
and 2% randomly in order to equal their personnel-constrained 8% limit. By
employing the algorithm described above, the terrorist cell knows that since
their operative has previously probed the system without a flag, CAPS likely
will not flag him again. In essence, the terrorist cell is able to relegate its
harmful activities outside of the 6% CAPS flag zone. Now, their operative only
has a 2% chance of calling up a thorough inspection. Compare this to the 8%
chance the terrorist would incur under the random system. It’s clear that
terrorist cells would therefore prefer airports fortified by CAPS.

Upon
reading this analysis, it’s natural to feel uneasy. How is it that such an
intuitive system can actually weaken security? For clarity, it is useful to
draw a distinction between individual terrorists and the coordinated activities
of an entire terrorist cell. It is entirely probable that even a rudimentary
CAPS profile can flag many individual terrorists, shrinking the viable pool of
recruits that a terrorist cell can send on a mission. But so long as the cell
as a unit can identify those members who slip through CAPS, even if they are
few in number, it has an enhanced probability of mounting a successful attack.
It only takes one person to do harm. Carnival Booth makes the process of
identifying such a person simple.

Evolutionary
biology validates this perspective. The most famous example is the peppered
moths of London. In the 1950s, Dr. H.B.D. Kettlewell, a physician, noticed that
the peppered moth population living near industrialized areas of London started
to change in color from light gray to dark gray. Through a series of
experiments, Kettlewell showed that dark colored moths were more apt to survive
near industrialized areas because, by matching the color of the smokestack soot
that coated the ground, they could evade predators.[17]
Individual light gray moths were eliminated from the gene pool, but since the
population had a few dark gray moths (maybe only one), the species survived and
proliferated. Even in nature, a non-conscious species can subvert static
environmental conditions that expose them to danger. Conscious terrorists, one
would expect, can likewise circumvent a profile.

Terrorists,
of course, are not moths. In fact, the evolutionary biology example points to
several assumptions we must make in order to claim the effectiveness of our
CAPS circumvention algorithm. Unlike a natural species, terrorists do not have,
for example, infinite variation and boundless evolutionary time. This leads to
three key questions: Do terrorist cells have a diverse enough membership to
successfully use this algorithm? Do they have the money, patience, and planning
skill to execute it? And above all, are they smart enough to know to use it?
Using findings from our nation’s “War on Terror” as a case study, we claim that
it is safe to assume that the answers to all of these questions are
affirmative.

The
terrorists revealed in recent months are strikingly diverse. Most famously,
John Walker Lindh— the “American Taliban”[18]—
is a nineteen-year-old Caucasian boy from Marin County, the yuppie capital of
California. He was fighting on the front lines for the Taliban in Afghanistan
against American soldiers. If he had never been discovered, would it have been
difficult for Al Qaeda to send him back to the United States on a terrorist mission?
Then there is “shoe bomber” Richard Reid, accused of trying to detonate
explosives in his sneakers during a transatlantic flight.[19]
He is a British citizen with an English mother and Jamaican father. And just
last week the FBI apprehended Lucas Helder, a 21-year-old art major at the
University of Wisconsin-Stout.[20]
Helder allegedly planted 18 pipe bombs in mailboxes in five different states to
form a “smiley face” pattern. And who can forget Ted Kaczynski and Timothy
McVeigh? Terrorists clearly have no shortage of diversity.

As the
hijackers of September 11 showed, they also have no shortage of money,
patience, and planning acumen. Mohammed Atta planned the attack years in
advance, studying diagrams of the World Trade Center, going to flight school,
and analyzing airplane specifications on the Internet. The money trail funding
Atta’s operation weaves through anonymous bank accounts in multiple countries.
It is so intricate that the FBI still does not completely understand it.
Terrorist cells like Al Qaeda have demonstrated that they have the resources
required in terms of money, patience, and planning to use the algorithm.

What may be
more alarming is that evidence from the September 11 investigation shows that
Atta already knew the kernel idea behind this algorithm. Newsweek
reported[21]
that in the weeks before September 11, Atta and his conspirators practiced
their attack by boarding the exact same target flights they intended to later
hijack (same planes, same times, same origins and destinations). They wanted to
ensure that they didn’t raise any suspicions or red flags. This is a clear
demonstration of Atta’s cleverness. Like Atta, terrorists are smart. They
already know this algorithm. And they are already using it.

A combination
of a probabilistic analysis and results from a computer simulation demonstrate
in more concrete terms if it is possible for a terrorist cell to use Carnival
Booth to defeat the CAPS system. In particular, we wanted to find out under
what conditions CAPS outperforms or underperforms random search, and we wanted
to quantitatively determine how much a terrorist cell could benefit from using
the algorithm. To accomplish this, we compared three systems: (1) the CAPS
system, (2) a system where passengers are randomly selected, and (3) a random
system with more advanced administrative searching. The results are clear. The
less a system relies on profiling and the more advanced its administrative
searching, the more terrorists it will catch.

The
analysis makes two reasonable assumptions. First, a terrorist must bring a
weapon or bomb onboard the airplane to cause damage. Modifications to cockpit
doors, sky marshals, and heightened passenger awareness after September 11th
have forced potential terrorists to use more than cardboard cutters to gain
control of a plane. Second, a future terrorist that is flying with no weapons
and no criminal intent will not be apprehended by law enforcement. Even if this
person has the highest threat index possible, there would be no reason to
apprehend him or her (barring an outstanding warrant or alleged connection to a
previous terrorist act).

We model
all three flavors of airport security with a two-stage architecture. The first
stage is administrative screening, which includes the usage of metal detectors
and basic questioning about luggage contents. All passengers are subjected to
this level of screening. The second stage is increased screening for those who
are either flagged by CAPS or randomly selected.

Using
probability theory, we developed a generalized function for these two-stage
systems that determines the probability that a terrorist will be caught. First,
the total probability of the terrorist being arrested is the sum of whether the
terrorist is arrested during a 2nd level search prompted randomly, a 2nd level
search prompted by a CAPS flag, or a 1st level administrative search:

Pr(Terrorist Arrested) =

Pr(Terrorist Arrested During
Randomly-flagged 2nd Level Search)

+
Pr(Terrorist Arrested During CAPS-flagged 2nd Level Search)

+
Pr(Terrorist Arrested During Administrative 1st Level Search)

Introducing some notation, let A be whether the terrorist is
arrested, R be whether the terrorist is randomly chosen for 2nd level
screening, and C be whether the terrorist is selected by CAPS for 2nd level
screening. Since the probability that the terrorist will undergo administrative
search is equal to the probability that the he is not flagged for 2nd level
search either by a CAPS flag or a random flag, the full equation becomes:

Pr(A) =

Pr(A | R) * Pr(R)

+ Pr(A | C)
* Pr(C)

+ Pr(A |
¬(R U C)) * Pr(¬(R U C))

To analyze
the dynamics of this equation, there are several constraints we can impose on
these probabilities. First observe that the probability that 2nd level
interrogation results in an arrest should be the same whether the terrorist is
flagged by the CAPS profile or randomly. Hence, Pr(A | R) must be the same as
Pr(A | C). Also, since the 2nd level searches are more thorough than 1st level
searches, logic dictates that Pr(A | ¬(R U C)) be lower than either Pr(A | R)
or Pr(A | C). Finally, in all the simulations, the number of passengers that
will be subjected to heightened security will be the same.

Although
information about the CAPS system is limited, the total percentage of people
flagged by CAPS, either through a profile or randomly, is reported to be
between 2-8%. Accordingly, we choose Pr(R) to be 2% under CAPS, and 8% under a
random system where all personnel resources can be devoted to random
inspections. Since second level searches are presumably more effective than
first level searches, our system arbitrarily sets Pr(A | R) at 75% and Pr(A | ¬
(R U C)) at 25% for the first two systems. For the third system employing better
administrative searches, Pr(A | ¬(R U C)) is set to 40%. We can begin to
compare the systems now that all values except Pr(C) are now known:

(CAPS system) Pr(A)
= 0.75 * 0.02 +

0.75
* Pr(C) +

0.25
* (1 - 0.02 - Pr(C))

(Random system) Pr(A)
= 0.75 * 0.08 +

0.75
* 0 +

0.25
* 0.92= 29%

(Random w/ admin+) Pr(A)
= 0.75 * 0.08 +

0.75
* 0 +

0.40
* 0.92 = 42.8%

The
critical value Pr(C) depends on the effectiveness of the CAPS profile and
whether terrorist cells are strategically using agents with low threat
indices.If Pr(C) is greater than 6%,
then the CAPS system will be more successful than a random system. If the
probability a terrorist will be flagged by CAPS is less than 6%, then law
enforcement should randomly select passengers for increased scrutiny. (The
third equation demonstrates how effective even modest improvements in
administrative searches can be. We will return to this topic later in our
discussion.)

Clearly, we
claim in this paper that by using our algorithm, a terrorist cell can push
Pr(C) under 6%. This probability heavily depends on how many probes an agent
completes before that terrorist cell sends the agent on a mission. Imagine that
a terrorist cell sends a new recruit to probe a domestic flight. On average
there could be 74 other passengers on that flight with him.[22]
Watching the other passengers closely at security checkpoints, the agent
notices that he has not been flagged. Although the cell can correctly conclude
that 6% of his peers on the flight (approximately 6 passengers) had threat
indices higher than his, this is no guarantee that on the next flight, the
agent will still be in the bottom 94% of threat indices. Thus, the terrorist
cell remains unsure whether they can “safely” send the agent on a mission.
However, the more probes that the agent returns from without attracting a CAPS
flag, the more confidence the cell has in this agent’s ability to get through
the system undetected.

With an
infinite number of probes, the CAPS system will perform worse than a random
system. If an agent has passed through security unflagged an infinite number of
times, then the probability the agent will be flagged on his mission, Pr(C),
becomes 0. Thus the overall probability of such an operative being apprehended
is:

(CAPS System)limitPr(A) = 0.75 * 0.02 + 0.75 * 0 +
0.25 * 0.98 = 26%

Probes -> Inf

This is four percentage points inferior to the random
system. Obviously, a terrorist cell cannot launch an infinite number of probes.
The chief question then becomes how many probes must a terrorist cell launch
before they can be sure that their operative stands a smaller chance of
apprehension than under a random system?

We
developed a computer simulation to shed light on how much more confident the
cell can be after each additional probe. Unfortunately, to definitively answer
this question one would need the distribution of CAPS scores for both
terrorists and the public at large. These distributions are, of course,
confidential.Hence we ran our
simulation several times using a wide variety of distributions, hoping that the
real distributions are bounded by our models.

In crafting
these distributions, we made a few general assumptions. The first is that
terrorists, on average, have a higher CAPS index than the average airline
flier. It would be unreasonable for the FBI to advocate a profiling-based
system if this assumption were false. The second assumption is that the
terrorist and general public distributions are Gaussian. This assumption stems
from the fact that CAPS scores are generated from thousands of boolean tests. A
combination of many random factors will usually create the bell-shaped Gaussian
distribution curve. While the actual distributions probably exhibit non-zero
skewness and kurtosis, we do not claim to know in which directions these
statistical moments would manifest themselves; we therefore assume they are
zero.

As for the
more significant moments of mean and standard deviation, we varied these values
in an attempt to cover many of the potential cases. Some distributions
represented moderately effective profiles with a mean CAPS score of 50 out of
100 for terrorists and 30/100 for non-terrorists and standard deviations of 20.
Other distributions modeled highly tuned profiles with a mean CAPS score of
70/100 for terrorists and 20/100 for non-terrorists and standard deviations of
20.

For each
distribution, and for each number of probes, 7,000 attacks were simulated. The
computer tracked a simulated terrorist cell as it used the Carnival Booth
algorithm to recruit new members and probe the system with each neophyte.
During a probe, the computer randomly generated 75 other non-terrorist
passengers. After finding a member that passed through the requisite number of
such probes unflagged, it sent that agent on a mission. On that mission, the
terrorist could either be flagged by CAPS, randomly selected, or neither,
depending on the CAPS

score probability distribution being tested. The percentage
of terrorists arrested was then recorded.

Figure 2: Number of Probes Required to
Defeat CAPS for Varying Score Distributions

As shown in
Figure 2, changes in the mean and standard deviation values make little difference
in the end result. If a terrorist cell requires four unflagged probes prior to
launching an attack, the CAPS model becomes worse than random. At six probes,
the threat index becomes an entirely useless means of identifying terrorists.
This immunity to the characteristics of the CAPS distribution is a striking
result— it means that even if CAPS utilized highly accurate profiles, a
terrorist operative need only launch a few successful probes before he can beat
the system.[23]

If CAPS is
not a viable means of strengthening security, the question remains how to make
our airports safer. With a random system, the paramount equation is:

(Random System)Pr(A)
= Pr(A | R) * Pr(R) + Pr(A | ¬R) * Pr(¬R)

The most effective security measure would be one that increases
the percentage of people receiving second level searches. But given that
passengers would object to the prohibitive waiting times, the government has
decided to inconvenience only 8 percent of airline passengers with increased
screening. Thus, we consider Pr(R) fixed at 8% and Pr(¬R) fixed at 92%. The
only two variables left to manipulate are the probability of catching a
terrorist during administrative screening and the probability of catching a
terrorist during heightened scrutiny. To maximize the equation, the term
multiplying Pr(¬R) is the most advantageous to increase. This term corresponds
to the administrative search accuracy. Since administrative searches cover
everyone, investment in technologies used in those searches will return more
bang for the buck.

Two
detailed case studies buttress the proposal that investments should be diverted
from profiling to administrative searches. First, we examine the failed attempt
to bomb Los Angeles International Airport just before the millennium. Then, the
security system of El Al is analyzed and used as a benchmark for great
security. The net result of these case studies points both to flaws in the CAPS
profiling system and the strengths of administrative searching.

In August
1999, Ahmed Ressam began to plot a terrorist attack against the United States
from his home in Montreal. Ressam decided to target Los Angeles International
Airport since he had sensed lax security when he had flown through the airport
in February 1999. By September, Ressam had found an accomplice who funded the
project, which enabled him to begin to procure chemicals and timing devices.
His plan was to plant an explosive in a suitcase, put the suitcase in an
inconspicuous luggage cart, set the timing device, and leave the scene. With
everything prepared, Ressam flew to Vancouver and then left for the United
States in a rental car on December 14th.[24]

At
approximately 6pm, Ressam boarded the ferryboat Coho in Victoria,
British Columbia, which would take him to Port Angeles, Washington. Before
being allowed to board the Coho, Ressam, like all other passengers, was
required to pass through a U.S. customs pre-screening checkpoint. Ressam
presented the customs official with a fake ID; he used the alias Benni Antoine
Noris.[25]
Even though Ressam was wanted by French and Canadian law enforcement, a
background check on the name Noris cleared since it was not one of Ressam’s
known aliases.[26]
Ressam even presented (in addition to a photo ID) a Costco card in the name of
Noris.[27]
Thus, Customs let Ressam board the ferry.

Upon
arriving in Port Angeles, Ressam was the last person to drive his car off of
the ferry.[28] A
Custom’s agent, Diana Dean, approached Ressam’s car and began questioning him.
Dean recalled, “I noticed during routine questioning that Ressam was acting in
a nervous and strange manner while answering routine questions. I decided to
perform a more thorough secondary examination.”[29]The “secondary examination” required that
Ressam exit his vehicle and that the car be fully searched. Ressam initially
refused to cooperate with the search and only reluctantly exited his car. When
he saw the officials had found his explosives, which were hidden in the trunk,
Ressam ran.[30]
Customs officials chased Ressam for several blocks before apprehending him.

Ressam’s
use of an alias demonstrates a weakness in profiling. Since the standard
identification in the United States is a state-issued driver’s license, there
are fifty different sets of rules. Terrorists can attack the weakest link. For
instance, in preparation for the September 11th attacks, seven of the nineteen
hijackers abused Virginia’s relaxed standards to obtain false licenses. Even if
America adopts a national ID, potential terrorists can acquire falsified papers
in other countries, as Ressam did.[31]

Saving the
day in this case were the security standards of the US Customs Department and
the professionalism of Diana Dean. No profile could have prevented Ressam from
entering the country—all his papers appeared to be in order. The experience of
Dean to perceive Ressam’s uneasiness and the searching procedures that found
the explosives concealed in the spare tire well of the truck prevented disaster
from striking one of America’s largest airports.

El Al is
Israel’s airline, and while it receives daily threats, they have not
experienced a terrorist incident in over thirty years. Its success is mainly
due to its tight on-site security. Each passenger each time he or she flies is psychologically
evaluated. Carryon bags are checked multiple times. In essence, El Al’s
security system works from the assumption that every passenger is a threat, and
treats him or her accordingly.

Every car
that enters Ben Gurion International Airport in Tel Aviv is examined.Plain-clothed guards help secure each airport
building.[32]
As El Al's president, David Hermesh, stated, “If you're a passenger on El Al,
most likely you will be observed from the minute that you left your car or you
have been dropped off.”[33]

Upon
entering the airport, a passenger is asked a series of specific questions
including “Who paid for your ticket?” “What is the purpose of your travels?”
and “When did you book the flight?”[34]
The questions are specifically designed to evoke an observable reaction. As
passengers answer the questions, the officer carefully scrutinizes tone of
voice, body language, and quickness of response. If the answers are
unsatisfactory, a different officer will ask the ticket holder a different set
of questions. Security personnel also might separate flight companions, making
sure their stories match.[35]
A psychological evaluation of the passenger is combined with information about
the passenger previously obtained by El Al and Interpol.[36]

On the
baggage side of the equation, El Al is just as strict. All luggage goes through
a decompression chamber designed to trick bombs that are set to go off when the
barometric pressure indicates that the plane is in the sky.[37]
Sophisticated X-ray technology is used to detect liquid explosives.[38]
Security personnel handle and examine every piece of carryon luggage. Every
piece of luggage is also matched to its owner. And on layover stops, passengers
must reclaim their luggage.[39]

While El Al
does keep a database of individuals’ nationalities, genders, criminal records
and flight histories, this tracking should not be confused with the a CAPS-like
profile.[40]
Unlike CAPS, El Al’s system only makes a determination of the risk of a
passenger after a security agent has questioned him or her.[41]
CAPS, on the other hand, is a prior system focused on predicting who will
become a terrorist. El Al, through its system of psychological analysis and
advanced baggage screening, has found success in determining who is a
terrorist.

Our
analytic result that CAPS-based airport searches are less effective than random
searches has important legal implications. While metal detectors and security
checkpoints at airports may seem routine to us today, it was not always so. It
was only after an extensive history of litigation that such searches were
deemed permissible[42].

Initially,
in light of the Fourth Amendment of the Constitution, it may not even be clear
why any airport searches are authorized. The Fourth Amendment protects
"the right of people to be secure in their persons, papers, and effects
against unreasonable searches and seizures." The keyword is
"unreasonable." Courts have generally recognized searches as being
reasonable if they properly balance the degree of intrusiveness, the magnitude
and frequency of the threat, and the efficacy of alternatives to the search.[43]
Through all of the litigation, courts have established two major exceptions to
the Fourth Amendment in relation to airport security: the administrative search
exception and the stop-and-frisk exception. Any new security technique
implemented at airports must conform to one of these two principles.

For a
search to qualify for the administrative search exception, it must serve a
societal purpose, be minimally intrusive, and all persons must be searched
equally no matter what level of suspicion they may arouse.[44]
An assortment of court cases establishes these criteria, but United States
v. Martinez-Fuerte[45]
(1976) provides the most direct example. In Martinez-Fuerte, the Supreme
Court upheld the practice of border control agents stopping vehicles at
checkpoints. First, the Court recognized that governmental interests may in
some situations trump Fourth Amendment protections. The Court wrote,
"Government or public interest in making such stops outweighs the
constitutionally protected interest of the private citizen." Furthermore,
the Court's opinion relied on the fact that the search was brief in duration,
usually consisting of only a couple of questions and visual inspection. Since
it did not greatly impede the flow of traffic or consume too much of the
traveler's time, the Court ruled that the search was minimal in its
intrusiveness given the government's compelling interest in border control.
Finally, the Court held "that the stops and questioning at issue may be
made in the absence of any individualized suspicion." Most importantly,
what made the border control stops permissible was that everyone, regardless of
suspicion, was democratically screened.[46]

Metal
detectors at airports are therefore justified under the administrative search
exception. By deterring people from bringing weapons onboard aircraft, they
serve a societal purpose. They are also minimally intrusive. And security
personnel require every passenger to go through them. CAPS-based screening, on
the other hand, does not fit all of the administrative search exception
criteria. In particular, the use of a profile to select particular individuals
for scrutiny violates the requirement that the search be equally applied to all
individuals regardless of suspicion.

The second
type of exception under which an airport security technique may qualify is the
stop-and-frisk exception. Under this type of exception, if security personnel
have a minimal level of justification that a person may pose danger, they may
conduct, without a warrant, a limited search of that person for the presence of
weapons. Like the administrative search exception, the stop-and-frisk exception
also has a complex history of precedent. Terry v. Ohio[47]
(1968) is the case that perhaps best defines this standard. In this case, a
Cleveland police officer patrolling his downtown beat observed two individuals
stare into a storefront window a total of 24 times. Suspecting that they might
be preparing to rob the store, the officer confronted the two men and frisked
them for weapons and found each of them carrying a revolver. The Supreme Court
was asked to decide whether the officer should have been permitted to frisk the
two men since he did not have a warrant. The Court decided that although the
officer had not established probable cause, his objective observations of the
men's suspicious behavior provided enough justification.[48]

Subsequent
court cases have further clarified what exactly constitutes a minimal level of
objective justification. In United States v. Cortez[49]
(1981), the Supreme Court decided that the "totality of
circumstances" may be considered in determining if suspicion exists.
Security personnel need more than a hunch to conduct a stop-and-frisk search,
but they need not establish probable cause. More precisely, given all of the
circumstances, the suspicion must be based upon an enhanced
"likelihood" that the person poses harm.[50]

On the surface,
CAPS-based security screening appears to qualify for the stop-and-frisk
exception. By targeting people fitting a suspicious profile, the FAA would
argue that CAPS establishes sufficient objective justification. However, based
on our analysis of CAPS, we reach the opposite conclusion. We showed that under
the CAPS system a terrorist can reduce his chance of arrest to a level lower
than a system that uses random search. This means that when security personnel
conduct their second level search of passengers flagged by CAPS, they actually
have a reduced probability of apprehending a terrorist carrying weapons
or explosives. Since random searches can catch more terrorists, airport
security cannot therefore legitimately establish that those passengers flagged
by CAPS have an enhanced likelihood of harm. Consequently, CAPS fails to meet
the standards of the stop-and-frisk exception.

Would it be
possible to convince a judge that CAPS as currently implemented is not
permissible for use in airports since it does not qualify for either the
administrative search or the stop-and-frisk Fourth Amendment exceptions? Our
analytic results fight an uphill battle against intuition. Although it is not
certain that a judge would strike down CAPS as being unconstitutional, we hope
that our analysis will encourage the judge to ask the FAA to be more convincing
about why CAPS is constitutional.

7Policy Recommendations

As the
evidence has shown, the most efficient approach to reducing terrorism is to
catch a terrorist in the midst of a mission. Attempting to determine who might
become a terrorist in the future or who acts like a terrorist is error-prone
and easy to defeat. As mathematically demonstrated in Section 4.1, the focus of
the FAA's efforts to tighten airport security should be augmenting
administrative searches that affect everyone that boards a flight.

The
questions that airline employees or security personnel ask passengers must
evoke more of a response. Currently, the standard questions are "Has your
luggage been in your possession at all times?" and "Has anyone given
you anything or asked you to carry on or check any items for them?" For
the experienced traveler, these questions induce the reflex answers of
"yes" and "no." By asking questions that require a slightly
longer response, security personnel would be able to gather information from
body language and tone of voice. This strategy also requires the monetary
investment to train the questioners so they know what signs to look for.

Techniques
for screening checked and carryon luggage have the advantage that they are hard
to probe. It is difficult to know whether an explosive will evade security
unless one brings an explosive through airport security. If a terrorist is
unsuccessful during a probe of such a system, they will be arrested for the
weapons they are carrying. Thus, a terrorist cell cannot probe administrative
searches of bags and persons. Increasing the effectiveness of object searching,
perhaps through new X-ray technology, should be a top priority.

If the US
government is determined to keep the CAPS system, then it is imperative that in
the future terrorists are prevented from defeating the system. Any new versions
of CAPS must reduce the Carnival Booth effect. In essence, it must be
non-obvious if a passenger has been flagged.

For checked
luggage this veil is easy to create since the passenger is unaware of what
occurs below the concourse. Bags that belong to individuals not considered
flight risks could be scanned with normal "medium x-ray" technology.
On the other hand, the luggage of flagged passengers could be scanned with a
slower, more detailed, and more costly "computer tomography" scanner.[51]

Disguising
the two-level process when screening carryon bags and the passengers themselves
is much more difficult. One possible solution could be to merge the new X-ray
technology for screening individuals (such as America Science and Engineering,
Inc.'s "Body Search"[52])
and the standard metal detectors into one device. This device could be a dual
metal detector and X-ray scanner. Similar to the checked baggage system, for
each ticket holder a security officer would discreetly turn on or off the X-ray
scanner.Thus, only "flight
risks" would be intrusively screened with the more costly X-ray scanner.

Barring
such sophisticated technology, CAPS must be dismantled. The carnival booth
effect prevents CAPS from being an effective deterrent against terrorism.
Effective deterrents are better-trained security personnel, more effective
questioning, and more advanced administrative screening. To truly make our
airways safer, these policies need to be applied to all passengers, not just
the few that get flagged for extra scrutiny. In short, the financial resources
our nation commits to counter-terrorism should not support a CAPS-like system,
which may appeal to our intuitions, but is in fact more amenable to compromise.

[Aaron wrote Sections 4, 5, 7 and wrote
the computer simulation.

Samidh served as the editor of this paper
and wrote Sections 1, 2, 3, 6.]

[1]
September 11, 2001 may have been the most widely covered American news story
ever. For a more thorough treatment of the events of that day, see:
http://www.time.com/time/911/

[8]
"Final Report to President Clinton." White House Commission on
Aviation Safety and Security. February 17, 1997. Available at:
http://www.airportnet.org/depts/regulatory/gorecom.htm

[9] “Hearing
on Aviation Security with a Focus on Passenger Profiling.” United States
Congress Subcommittee on Aviation. February 27, 2002. Available at:
http://www.house.gov/transportation/aviation/02-27-02/02-27-02memo.html

[10] For
more information, see HNC Software’s website: http://www.hnc.com